Aerosol-generating article having a thermal indicator

ABSTRACT

An aerosol-generating article for an aerosol-generating device having a heating element is provided, the article including: a rod of aerosol-generating substrate formed of an aerosol-forming material having an aerosol former content of between 5 percent and 30 percent on a dry weight basis; and an outer wrapper wrapped around at least a portion of the rod, and at least a portion of an outer surface of the wrapper includes a thermal indicator including a thermochromic material, at least a portion of which is configured to undergo a visual change when a temperature at the portion exceeds a threshold temperature, such that the outer wrapper provides a visual indication that a temperature at the thermal indicator has exceeded the threshold temperature, the threshold temperature being at least about 175° C.

The present invention relates to a heated aerosol-generating articleincorporating a thermal indicator comprising a thermochromic material,to the use of such thermochromic material in such a heatedaerosol-generating article and to an aerosol-generating systemcomprising such a heated aerosol-generating article having a thermalindicator.

Aerosol-generating articles in which an aerosol-generating substrate,such as a tobacco-containing substrate, is heated rather than combusted,are known in the art. Typically in such heated smoking articles, anaerosol is generated by the transfer of heat from a heat source to aphysically separate aerosol-generating substrate or material, which maybe located in contact with, within, around, or downstream of the heatsource. During use of the aerosol-generating article, volatile compoundsare released from the aerosol-generating substrate by heat transfer fromthe heat source and are entrained in air drawn through theaerosol-generating article. As the released compounds cool, theycondense to form an aerosol.

A number of prior art documents disclose aerosol-generating devices forconsuming or smoking heated aerosol-generating articles. Such devicesinclude, for example, electrically heated aerosol-generating devices inwhich an aerosol is generated by the transfer of heat from one or moreelectrical heating elements of the aerosol-generating device to theaerosol-generating substrate of a heated aerosol-generating article. Oneadvantage of such electrically heated aerosol-generating devices is thatthey significantly reduce sidestream smoke.

In such aerosol-generating devices, the heating element will typicallybe configured to heat the aerosol-generating substrate within a definedtemperature range which has been selected by the manufacturer to providean optimal aerosol release profile from the aerosol-generating article.The aerosol-generating article and aerosol-generating device aretherefore specifically adapted for use in conjunction with each other.

However, where an aerosol-generating article is inadvertently orintentionally used with an incompatible aerosol-generating device, anoptimal aerosol release profile is very unlikely to be provided to theconsumer. The construction of the heating element in the incompatibledevice will typically be different to that of the compatible device andthe form of heating of the aerosol-generating substrate is thereforelikely to be different. Furthermore, the heater may not be operated inthe same way within the same temperature ranges, so that theaerosol-generating substrate will not be heated under the sametemperature profile as in a compatible device. As a result, theproperties of the aerosol released from the substrate will not be asintended by the manufacturer. The experience of the consumer will likelytherefore be adversely affected as a result of using theaerosol-generating article with an incompatible device.

Particular problems may arise when an aerosol-generating article is usedin a device that heats the aerosol-generating substrate to a highertemperature than intended, such that at least a part of the substratebecomes overheated. This may occur, for example, when anaerosol-generating article that is adapted to be heated by an internalheating element is instead used in an aerosol-generating device thatheats the aerosol-generating article externally. Such devices that heatthe substrate from the outside during use typically require much higheroperating temperatures and therefore at least the outer parts of thesubstrate are likely to be heated to a much higher temperature thanwould be provided using an internal heating element. Additionally,overheating of the substrate may also occur from internal heating of theaerosol-generating article to also much higher operating temperatures bya compatible or in-compatible aerosol-generating device that heats theaerosol-generating article internally.

It would be desirable to provide a novel arrangement of anaerosol-generating article which provides a visual indication or warningto a user of the use of the aerosol-generating article with anincompatible aerosol-generating device and in particular, with anyaerosol-generating device that heats the aerosol-generating substrate toa higher temperature than is intended for the substrate. It would befurther desirable to provide such a novel arrangement of anaerosol-generating article which does not adversely impact the use ofthe aerosol-generating article under normal heating conditions in acompatible device. It would be particularly desirable if such a novelarrangement of an aerosol-generating article could be readily providedwithout significantly impacting the construction of theaerosol-generating article or the method and apparatus used for theproduction of the aerosol-generating article.

According to a first aspect of the present invention, there is provideda heated aerosol-generating article for use with an aerosol-generatingdevice having a heating element, the heated aerosol-generating articlecomprising: a rod of aerosol-generating substrate; and an outer wrapper,wherein the outer wrapper is wrapped around at least a portion of therod of aerosol-generating substrate and at least a portion of an outersurface of the wrapper comprises a thermal indicator. The thermalindicator comprises a thermochromic material, whereby at least a portionof the thermochromic material is adapted to undergo a visual change whenthe temperature at said portion of the thermochromic material exceeds athreshold temperature, such that the outer wrapper provides a visualindication that the temperature at the thermal indicator has exceededthe threshold temperature, wherein the threshold temperature is at leastabout 175° C.

According to a second aspect of the present invention, there is provideda heated aerosol-generating article for use with an aerosol-generatingdevice having a heating element, the heated aerosol-generating articlecomprising: a rod of aerosol-generating substrate; and an outer wrapper,wherein at least a portion of an outer surface of the wrapper comprisesa thermal indicator, wherein the thermal indicator is located downstreamof the rod of aerosol-generating substrate. The thermal indicatorcomprises a thermochromic material, whereby at least a portion of thethermochromic material is adapted to undergo a visual change when thetemperature at said portion of the thermochromic material exceeds athreshold temperature, such that the outer wrapper provides a visualindication that the temperature at the thermal indicator has exceededthe threshold temperature, wherein the threshold temperature is at leastabout 115° C.

According to a third aspect of the present invention, there is provideda use of thermochromic material in a thermal indicator located on anouter wrapper of a rod of aerosol-generating substrate of a heatedaerosol-generating article, the heated aerosol-generating article beingconfigured to be used with an aerosol-generating device having a heatingelement, wherein the outer wrapper is wrapped around at least a portionof the rod of aerosol-generating substrate and whereby at least aportion of the thermochromic material is adapted to undergo a visualchange when the temperature at said portion of the thermochromicmaterial exceeds a threshold temperature, such that the outer wrapperprovides a visual indication that the temperature at the thermalindicator has exceeded the threshold temperature, wherein the thresholdtemperature is at least about 175° C.

According to a fourth aspect of the present invention, there is provideda use of thermochromic material in a thermal indicator located on anouter wrapper of a heated aerosol-generating article, the heatedaerosol-generating article being configured to be used with anaerosol-generating device having a heating element and the heatedaerosol-generating article comprising a rod of aerosol-generatingsubstrate, whereby the thermal indicator is located downstream of therod of aerosol-generating substrate and at least a portion of thethermochromic material is adapted to undergo a visual change when thetemperature at said portion of the thermochromic material exceeds athreshold temperature, such that the outer wrapper provides a visualindication that the temperature at the thermal indicator has exceededthe threshold temperature, wherein the threshold temperature is at leastabout 115° C.

According to a fifth aspect of the present invention, there is providedan aerosol-generating system comprising: a heated aerosol-generatingarticle according to the first or second aspects of the presentinvention as described above; and an aerosol-generating device adaptedto receive the heated aerosol-generating article. The aerosol-generatingdevice comprises a heater element configured to heat the rod ofaerosol-generating material during use wherein the heater element iscontrolled during use to operate below a maximum operating temperature.The thermal indicator of the heated aerosol-generating article isadapted such that the threshold temperature is not exceeded during useof the aerosol-generating system with the heater element operating belowthe maximum operating temperature.

As used herein, the term “heated aerosol-generating article” refers toan aerosol-generating article for producing an aerosol comprising anaerosol-generating substrate that is intended to be heated rather thancombusted in order to release volatile compounds that can form anaerosol. Such articles are commonly referred to as “heat-not-burn”products.

As used herein, the term “aerosol-generating substrate” refers to asubstrate capable of releasing upon heating volatile compounds, whichcan form an aerosol. The aerosol generated from aerosol-generatingsubstrates of aerosol-generating articles described herein may bevisible or invisible and may include vapours (for example, fineparticles of substances, which are in a gaseous state, that areordinarily liquid or solid at room temperature) as well as gases andliquid droplets of condensed vapours.

As used herein, the term “rod” refers to a generally cylindrical elementof substantially polygonal cross-section and preferably of circular,oval or elliptical cross-section. The term “article” will refer to anaerosol-generating article, unless otherwise stated.

As used herein, the term “longitudinal” refers to the directioncorresponding to the main longitudinal axis of the aerosol-generatingarticle, which extends between the upstream and downstream ends of theaerosol-generating article. During use, air is drawn through theaerosol-generating article in the longitudinal direction. The term“transverse” refers to the direction that is perpendicular to thelongitudinal axis.

As used herein, the terms “upstream” and “downstream” describe therelative positions of elements, or portions of elements, of theaerosol-generating article in relation to the direction in which theaerosol is transported through the aerosol-generating article duringuse.

Aerosol-generating articles according to the present invention aresuitable for use in an aerosol-generating system comprising anelectrically heated aerosol-generating device having an internal heatingelement for heating the aerosol-generating substrate. For example,aerosol-generating articles according to the invention find particularapplication in aerosol-generating systems comprising an electricallyheated aerosol-generating device having an internal heater blade whichis adapted to be inserted into the rod of aerosol-generating substrate.Aerosol-generating articles of this type are described in the prior art,for example, in European patent application EP-A-0 822 670.

As used herein, the term “aerosol-generating device” refers to a devicecomprising a heating element that interacts with the aerosol-generatingsubstrate of the aerosol-generating article to generate an aerosol.

As described above, aerosol-generating articles according to the presentinvention incorporate an outer wrapper comprising a thermal indicator.The thermal indicator provides an effective means for warning andpreventing use of an aerosol-generating article according to the presentinvention in an incompatible device that heats the aerosol-generatingarticle excessively above a desired operating temperature. Uponoverheating of the rod of aerosol-generating substrate, the thermalindicator will produce a visual change to warn the user of incorrectand/or potentially harmful use, which the user may see during or afteruse of the aerosol-generating article. In this way the user will bealerted to the fact that they have used or are using theaerosol-generating article with an incompatible device, or with acompatible device that is malfunctioning and overheating theaerosol-generating article during use. The thermal indicator thereforeprovides means for prevention of overheating of the aerosol-generatingarticle.

As used herein with reference to the invention, the term “thermochromicmaterial” is used to describe any material that undergoes a visualchange at a predetermined threshold temperature or range oftemperatures.

Thermochromic material may undergo a visual change as the temperaturerises above or falls below the threshold temperature. As used hereinwith reference to the invention, the term “threshold temperature” isused to describe the temperature after which the visual change of athermochromic material is substantially complete.

Advantageously, the thermal indicator is adapted so that it will undergoa visual change at excessive operating temperatures. Therefore, when theaerosol-generating articles according to the present invention areheated normally in a compatible aerosol-generating device, the presenceof the thermal indicator will not perceivably impact the consumerexperience. In particular, the presence of the thermal indicator doesnot adversely impact the resistance to draw (RTD) of theaerosol-generating article.

The outer wrapper comprising a thermal indicator can be convenientlyincorporated into an aerosol-generating article without significantlyimpacting the arrangement of the other components of the article. Theinclusion of the thermal indicator should not therefore significantlyimpact the manufacture of the aerosol-generating articles.Aerosol-generating articles according to the invention can thereforeadvantageously be made using existing high speed methods and apparatus,with only minor modifications.

The positioning of the thermal indicator on the outer wrapper,surrounding the outside of the aerosol-generating article, and thematerial selected for the thermal indicator determine the responsivenessof the thermal indicator to overheating of the aerosol-generatingarticle due to use in an incompatible device having, for example, aperipheral heater that heats the aerosol-generating substrateexternally.

In the event that the aerosol-generating article is overheated, forexample in an incompatible device, so that the thermal indicator reachesits threshold temperature, the thermal indicator will be “activated” andundergo a visual change. The activation of the thermal indicator willtherefore provide an effective means for warning and preventing use ofan aerosol-generating article according to the present invention in anincompatible device that heats the aerosol-generating articleexcessively above a set of desired operating temperatures and above thethreshold temperature.

The “threshold temperature” of the thermal indicator corresponds to thetemperature at which the thermal indicator will undergo a visual change,as described above. This will typically correspond to the thresholdtemperature of the material used to form the thermal indicator. Asdiscussed in more detail below, the threshold temperature of and at thethermal indicator will be selected so that it will only be reached orexceeded when the aerosol-generating article is overheated, that is,heated above the intended operating temperature range. The thresholdtemperature will therefore correspond to the temperature reached at thethermal indicator when the aerosol-generating substrate is heated abovethe maximum desired temperature, as determined by the manufacturer.

There are several potential ways in which the thermal indicator may beadapted so that activation occurs at the desired threshold temperatureand so that the visual change is produced at the thermal indicator toprevent further use of the aerosol-generating article.

A suitable material should be selected for forming the thermalindicator, wherein the material has an appropriate activation point orthreshold temperature to ensure that there is no risk of the thermalindicator undergoing a visual change when the aerosol-generating articleis heated to within a normal operating temperature range but also sothat the thermal indicator does effectively undergo a visual change whenthe aerosol-generating article is overheated to a temperature above thisrange. This would avoid erroneous indications of overheating when theaerosol-generating indicator operates within a normal operatingtemperature range and provide a reliable overheating indicator when theaerosol-generating indicator does operate outside or above a normaloperating temperature range.

A suitable position of the thermal indicator on the aerosol-generatingarticle will also need to be selected. The appropriate position for thethermal indicator will depend to an extent upon the selected material sothat the threshold temperature can be defined appropriately. This willavoid the unintended activation of the thermal indicator during normaluse of the aerosol-generating article.

Typically, during use, the aerosol-generating article will be insertedinto an aerosol-generating device such that the aerosol-generatingsubstrate is heated, either directly or indirectly, by a heating elementwithin the device. The temperature within the aerosol-generating articlewill be highest adjacent to the heating element and will decreaselongitudinally along the aerosol-generating article with increasingdistance from the heating element. The appropriate threshold temperaturefor the thermal indicator will therefore vary depending on how far thethermal indicator is placed downstream from the aerosol-generatingsubstrate. In particular, the further the thermal indicator is placeddownstream of the aerosol-generating substrate, the lower the thresholdtemperature will need to be in order for the thermal indicator toactivate.

The temperature profile within and surrounding an aerosol-generatingarticle can be readily measured using thermocouples such that anappropriate threshold temperature can be determined to ensure that thethermal indicator is activated only when the substrate is deemed to beoverheated, above a defined maximum temperature.

The thermal indicator may be suitably located anywhere on the outersurface of the outer wrapper. The thermal indicator may at least beadjacent to and/or around the rod of aerosol-generating substrate.

As discussed above, the threshold temperature of the thermal indicatoraccording to the present invention is selected depending on its locationalong the aerosol-generating article, which has a preferred range ofoperating temperatures.

Advantageously, the threshold temperature may be below about 300° C.

Advantageously, the threshold temperature may be between about 115° C.and about 300° C. For example, the threshold temperature may be betweenabout 120° C. and about 300° C. For example, the threshold temperaturemay be between about 155° C. and about 300° C. For example, thethreshold temperature may be between about 160° C. and about 300° C. Forexample, the threshold temperature may be between about 170° C. andabout 300° C. For example, the threshold temperature may be betweenabout 175° C. and about 300° C.

Advantageously, the threshold temperature may be below about 275° C.

Advantageously, the threshold temperature may be between about 115° C.and about 275° C. For example, the threshold temperature may be betweenabout 120° C. and about 275° C. For example, the threshold temperaturemay be between about 155° C. and about 275° C. For example, thethreshold temperature may be between about 160° C. and about 275° C. Forexample, the threshold temperature may be between about 170° C. andabout 275° C. For example, the threshold temperature may be betweenabout 175° C. and about 275° C.

Advantageously, the threshold temperature may be below about 250° C.

Advantageously, the threshold temperature may be between about 115° C.and about 250° C. For example, the threshold temperature may be betweenabout 120° C. and about 250° C. For example, the threshold temperaturemay be between about 155° C. and about 250° C. For example, thethreshold temperature may be between about 160° C. and about 250° C. Forexample, the threshold temperature may be between about 170° C. andabout 250° C. For example, the threshold temperature may be betweenabout 175° C. and about 250° C.

In accordance with the first aspect of the invention, the thermalindicator may be located around at least a portion of the rod ofaerosol-generating substrate. The thermal indicator may circumscribe andcover the entirety of the surface area of rod of aerosol-generatingsubstrate. Alternatively or in addition, the thermal indicator maycircumscribe at least a portion of the rod of aerosol-generatingsubstrate and, at least, another downstream portion of theaerosol-generating article. Alternatively, the thermal indicator maycircumscribe only a portion of the overall circumference or periphery ofthe aerosol-generating article, preferably around the rod ofaerosol-generating substrate.

In accordance with the first aspect of the invention, the thermalindicator is provided over the rod of aerosol-generating substrate andthe threshold temperature is preferably at least about 175° C., morepreferably about 180° C. and most preferably about 185° C. Preferably,the threshold temperature is at least about 220° C. The thresholdtemperature may be no more than about 300° C. This ensures that thethermal indicator is sufficiently sensitive to heating of theaerosol-generating substrate above the maximum desired temperature. Insuch embodiments, the threshold temperature of the thermal indicatorwill need to be relatively high due to the likely proximity to theheating element of the aerosol-generating device during use.

In accordance with a second aspect of the present invention, the thermalindicator may be located downstream from the rod of aerosol-generatingsubstrate, or the “substrate/distal end” of the aerosol-generatingarticle. As used herein with reference to the invention, the term“substrate/distal end” refers to the upstream end of theaerosol-generating article, also being the end of the aerosol-generatingsubstrate which is configured to be heated. The “mouthpiece end” of theaerosol-generating article is the downstream end of theaerosol-generating article, opposite the “substrate/distal end”.

In accordance with the second aspect of the invention, the thermalindicator, or a portion thereof, is provided downstream of the rod ofaerosol-generating substrate, the threshold temperature is preferably atleast about 115° C., more preferably about 120° C. and most preferablyabout 125° C. The threshold temperature may be no more than about 200°C. This ensures that the thermal indicator is sufficiently sensitive toheating of the aerosol-generating substrate above the maximum desiredtemperature. In such embodiments, the threshold temperature of thethermal indicator will need to be lower due to the likely greaterdistance to the heating element of the aerosol-generating device duringuse.

In accordance with all aspects of the present invention, the thermalindicator extends longitudinally along at least part of the length ofthe aerosol-generating article. For example, the length of the thermalindicator may be between about 1 mm and 20 mm. Moreover, the length ofthe thermal indicator may be between about 1 mm and 15 mm.

The thermal indicator may extend longitudinally along at least 20% ofthe length of the aerosol-generating article, more preferably at least25% of the length of the aerosol-generating article. Preferably, thethermal indicator may extend longitudinally along at least 50% of thelength of the aerosol-generating article.

The length of the thermal indicator may be the same length as theaerosol-generating article, or may be the same length as the rod ofaerosol-generating substrate. Preferably, the length of the thermalindicator may be at least 75% of the length of the rod ofaerosol-generating substrate, and more preferably at least 50% of thelength of the rod of aerosol-generating substrate.

The thermal indicator may extend circumferentially at least partiallyaround the aerosol-generating article. Preferably, the thermal indicatorextends around the entire circumference of the aerosol-generatingarticle.

In accordance with the first aspect of the invention, the thermalindicator may overlie at least 1 mm² of the external surface area of therod of aerosol-generating substrate. Preferably, the thermal indicatormay overlie at least 5 mm² of the external surface area of the rod ofaerosol-generating substrate.

In accordance with the first aspect of the invention, the thermalindicator may overlie at least 20% of the external surface area of therod of aerosol-generating substrate. Preferably, the thermal indicatormay overlie at least 25% of the external surface area of the rod ofaerosol-generating substrate. Preferably, the thermal indicator mayoverlie at least 50% of the external surface area of the rod ofaerosol-generating substrate.

In certain embodiments according to the first aspect of the invention,the thermochromic material may be arranged such that the visual changeoccurs only within a circumferential portion of the outer wrapper,wherein the circumferential portion may extend only partially around thecircumference of the aerosol-generating article. In such embodiments,the thermal indicator may be adapted to provide a localised visualchange, at a specific circumferential position around theaerosol-generating article, preferably at a specific circumferentialposition around the rod of aerosol-generating substrate. For example, acircumferential portion of the outer surface of the wrapper may comprisethe thermal indicator, or a circumferential portion of the thermalindicator may be configured to undergo a visual change, or both. Thecircumferential portion may extend at least 5% of the circumference ofthe article, more preferably at least 10% of the circumference of thearticle and most preferably at least 20% of circumference of thearticle. In such embodiments, the threshold temperature is preferably atleast about 175° C., more preferably about 180° C. and most preferablyabout 185° C. Preferably, the threshold temperature is at least about220° C. The threshold temperature may be no more than about 300° C. Thisensures that the thermal indicator is sufficiently sensitive tooverheating of the aerosol-generating substrate above the maximumdesired temperature. The temperature distribution along and/or acrossthe rod of substrate when heated may not be uniform and therefore localportions of the outer surface of the rod of aerosol-generating substratemay be overheated and other portions may not be overheated. In suchembodiments, the threshold temperature of the thermal indicator willalso need to be relatively high due to the likely proximity the thermalindicator to the heating element of the aerosol-generating device duringuse. The localised visual change may also provide an indication of, inaddition to localised overheating of the substrate, the alignment of aheating element of an aerosol-generating device with respect to theouter wrapper and rod of aerosol-generating substrate. If the heatingelement is configured to heat the substrate internally, then thethreshold temperature may be such that the localised visual change mayindicate that the region of the outer wrapper where the localised visualchange has occurred is likely to be closest to the heat element.

As discussed above, for all aspects of the invention, the thermalindicator is configured to undergo a visual change when a thresholdtemperature is exceeded. As discussed above, the visual change may occurover a portion of or the entirety of the thermal indicator depending onwhich portions of the thermal indicator have exceeded their respectivethreshold temperatures. The visual change may involve changing from afirst colour to a second colour or from substantially colourless to afirst colour or from a first colour to substantially colourless. Thevisual change may involve changing from a first colour, to a secondcolour and then to a third colour. The visual change may involvetransitioning from substantially colourless through a plurality ofdifferent colours to a final colour. The visual change may involvetransitioning from an initial colour through a plurality of differentcolours to a final colour. Each colour may provide a particularindication of the degree of overheating in the substrate. The visualchange may comprise the thermal indicator not displaying any warningindicia or messages to displaying warning indicia or messages. Thewarning indicia or messages may comprise logos or any form of graphicalrepresentation.

The visual change may be instantaneous or gradual over a broadtransition range of colour.

Advantageously, the visual change may be irreversible or reversible,more preferably irreversible. The visual change of the thermal indicatorbeing irreversible or substantially permanent allows the thermalindicator to suitably convey the message of misuse of theaerosol-generating article to the user at a time after the visual changeoccurs. The temperature at the thermal indicator may exceed thethreshold temperature before the user or consumer finishes with thearticle. Therefore, an irreversible visual change is advantageous forensuring that the user is informed of the misuse after they finish withthe article.

The visual change may encompass any combination of characteristicsdescribed above.

The thermal indicator may comprise a single thermochromic material witha specific threshold temperature and visual change. Alternatively, thethermal indicator may comprise a plurality of different thermochromicmaterials, wherein each of the different thermochromic materials maycomprise a different threshold temperature and/or visual change to eachother.

The thermal indicator may comprise a single thermochromic materialwherein the threshold temperature is uniform across and along thethermal indicator. Alternatively, the thermal indicator may comprise asingle thermochromic material wherein the threshold temperature varieseither longitudinally and/or along the thermal indicator. For example,the threshold temperature may decrease or increase in the downstream orupstream direction along the thermal indicator.

In all embodiments of the present invention, the thermal indicator isformed of thermochromic material. The thermochromic material may beapplied or affixed onto the surface of the outer wrapper, in the form ofpaint, ink, dye or another layer of material having thermochromicmaterial applied thereon. Alternatively, the outer wrapper may comprisetemperature indicating sensor paper, whereby the thermochromic materialis integral to the temperature indicating sensor paper. Thethermochromic material may further be integral to at least any othercomponent, layer or portion of the outer wrapper.

THERMEX® by Sensor Products Inc. (http://www.sensorprod.com/thermex.php)is an example of an irreversible thermochromic material suitable for usein providing the thermal indicator of the present invention. THERMEX® isa tactile surface temperature indicating material provided as sheetmaterial and configured to be placed between two contacting surfaces,and upon exposure to a particular threshold temperature or range ofthreshold temperatures, the THERMEX® sheet instantaneously andpermanently changes colour. The colour change is permanent orirreversible and the operational range of threshold temperatures is of90 to 149° C. Other suitable irreversible thermochromic materialsgenerally known to the skilled person may be used.

The thermal indicator may comprise a combination of reversible andirreversible thermochromic materials. In such cases, the thermalindicator may undergo both reversible and irreversible visual changes.Suitable reversible thermochromic materials generally known to theskilled person may be used.

As discussed before, the thermal indicator could be readily providedwithout significantly impacting the construction of theaerosol-generating article or the method and apparatus used for theproduction of the aerosol-generating article. The thermal indicator maytherefore easily be disposed on or applied or affixed to the innerand/or outer surfaces of the outer wrapper, preferably to or on theouter surface of the outer wrapper. The thermal indicator may bedisposed between two layers of wrapping material which may form theouter wrapper. The thermal indicator may be incorporated into the outerwrapper in a separate process so that the process of assembling theouter wrapper around the article is unaffected.

The aerosol-generating articles according to the present invention maycomprise a plurality of elements, including the rod ofaerosol-generating substrate and the thermal indicator located on theouter wrapper of the article. The outer wrapper may comprise a cigarettepaper.

Aerosol-generating articles commonly comprise a hollow acetate tubedirectly adjacent to the aerosol-generating substrate.

The rod of aerosol-generating substrate is formed of an aerosol-formingmaterial, which is particularly preferably homogenised tobacco material.

As used herein, the term “homogenised tobacco material” encompasses anytobacco material formed by the agglomeration of particles of tobaccomaterial. Sheets or webs of homogenised tobacco material are formed byagglomerating particulate tobacco obtained by grinding or otherwisepowdering of one or both of tobacco leaf lamina and tobacco leaf stems.In addition, homogenised tobacco material may comprise a minor quantityof one or more of tobacco dust, tobacco fines, and other particulatetobacco by-products formed during the treating, handling and shipping oftobacco. The sheets of homogenised tobacco material may be produced bycasting, extrusion, paper making processes or other any other suitableprocesses known in the art.

In preferred embodiments, the rod comprises one or more sheets of ahomogenised tobacco material that have been gathered to form a plug andcircumscribed by an outer wrapper. As used herein with reference to theinvention, the term “sheet” describes a laminar element having a widthand length substantially greater than the thickness thereof. As usedherein with reference to the invention, the term “gathered” describes asheet that is convoluted, folded, or otherwise compressed or constrictedsubstantially transversely to the longitudinal axis of theaerosol-generating article.

Advantageously, the aerosol-generating substrate comprises a gatheredtextured sheet of homogenised tobacco material. As used herein withreference to the invention, the term “textured sheet” describes a sheetthat has been crimped, embossed, debossed, perforated or otherwisedeformed.

Use of a textured sheet of homogenised tobacco material mayadvantageously facilitate gathering of the sheet of homogenised tobaccomaterial to form the aerosol-generating substrate.

The aerosol-generating substrate may comprise a gathered textured sheetof homogenised tobacco material comprising a plurality of spaced-apartindentations, protrusions, perforations or any combination thereof.

In certain preferred embodiments, the aerosol-generating substratecomprises a gathered crimped sheet of homogenised tobacco material. Asused herein with reference to the invention, the term “crimped sheet”describes a sheet having a plurality of substantially parallel ridges orcorrugations. Advantageously, when the aerosol-generating article hasbeen assembled, the substantially parallel ridges or corrugations extendalong or parallel to the longitudinal axis of the aerosol-generatingarticle. This facilitates gathering of the crimped sheet of homogenisedtobacco material to form the aerosol-generating substrate.

However, it will be appreciated that crimped sheets of homogenisedtobacco material for inclusion in the aerosol-generating substrates ofaerosol-generating articles according to the invention may alternativelyor in addition have a plurality of substantially parallel ridges ororrugations that are disposed at an acute or obtuse angle to thelongitudinal axis of the aerosol-generating article when theaerosol-generating article has been assembled.

Sheets of homogenised tobacco material for use in the invention may havea tobacco content of at least about 40 percent by weight on a dry weightbasis, more preferably of at least about 50 percent by weight on a dryweight basis more preferably at least about 70 percent by weight on adry weight basis and most preferably at least about 90 percent by weighton a dry weight basis.

Preferably, the sheets of homogenised tobacco material comprise anaerosol former. The sheets of homogenised tobacco material may comprisea single aerosol former. Alternatively, the sheets of homogenisedtobacco material may comprise a combination of two or more aerosolformers.

Suitable aerosol-formers are known in the art and include, but are notlimited to: monohydric alcohols like menthol, polyhydric alcohols, suchas triethylene glycol, 1,3-butanediol and glycerine; esters ofpolyhydric alcohols, such as glycerol mono-, di- or triacetate; andaliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyldodecanedioate, dimethyl tetradecanedioate, erythritol, 1,3-butyleneglycol, tetraethylene glycol, triethyl citrate, propylene carbonate,Ethyl laurate, triacetin, meso-erythritol, a diacetin mixture, a diethylsuberate, triethyl citrate, benzyl benzoate, benzyl phenyl acetate,ethyl vanillate, tributyrin, lauryl acetate, lauric acid, myristic acid,and propylene glycol.

Preferably, the sheets of homogenised tobacco material have an aerosolformer content of greater than 5 percent on a dry weight basis.

The sheets of homogenised tobacco material may have an aerosol formercontent of between approximately 5 percent and approximately 30 percenton a dry weight basis.

In a preferred embodiment, the sheets of homogenised tobacco materialhave an aerosol former content of approximately 20 percent on a dryweight basis.

Sheets of homogenised tobacco material for use in the invention maycomprise one or more intrinsic binders, that is tobacco endogenousbinders, one or more extrinsic binders, that is tobacco exogenousbinders, or a combination thereof to help agglomerate the particulatetobacco. Alternatively, or in addition, sheets of homogenised tobaccomaterial for use in the aerosol-generating substrate may comprise otheradditives including, but not limited to, tobacco and non-tobacco fibres,aerosol-formers, humectants, plasticisers, flavourants, fillers, aqueousand non-aqueous solvents and combinations thereof.

Suitable extrinsic binders for inclusion in sheets of homogenisedtobacco material for use in the invention are known in the art andinclude, but are not limited to: gums such as, for example, guar gum,xanthan gum, arabic gum and locust bean gum; cellulosic binders such as,for example, hydroxypropyl cellulose, carboxymethyl cellulose,hydroxyethyl cellulose, methyl cellulose and ethyl cellulose;polysaccharides such as, for example, starches, organic acids, such asalginic acid, conjugate base salts of organic acids, such assodium-alginate, agar and pectins; and combinations thereof.

Suitable non-tobacco fibres for inclusion in sheets of homogenisedtobacco material for use in the aerosol-generating substrate are knownin the art and include, but are not limited to: cellulose fibers;soft-wood fibres; hard-wood fibres; jute fibres and combinationsthereof. Prior to inclusion in sheets of homogenised tobacco materialfor use in the aerosol-generating substrate, non-tobacco fibres may betreated by suitable processes known in the art including, but notlimited to: mechanical pulping; refining; chemical pulping; bleaching;sulfate pulping; and combinations thereof.

Sheets of homogenised tobacco for use in the invention preferably have awidth of between about 70 mm and about 250 mm, for example between about120 mm and about 160 mm. Preferably, the thickness of the sheets ofhomogenised tobacco material is between about 50 micrometres and about300 micrometres, more preferably between about 150 micrometres and about250 micrometres.

Sheets of homogenised tobacco for use in the aerosol-generating articleof the present invention may be made by methods known in the art, forexample the methods disclosed in International patent applicationWO-A-2012/164009 A2.

In a preferred embodiment, sheets of homogenised tobacco material foruse in the aerosol-generating article are formed from a slurrycomprising particulate tobacco, guar gum, cellulose fibres and glycerineby a casting process.

As an alternative to the use of a gathered sheet of homogenised tobaccomaterial, as described above, the aerosol-generating substrate may beformed of a plurality of strips or shreds of a sheet of homogenisedtobacco material. For example, the aerosol-generating substrate may beformed of a plurality of shreds of homogenised tobacco material that arealigned in the longitudinal direction and have been brought together andwrapped to form a rod of aerosol-generating substrate.

The shreds of homogenised tobacco material preferably have a length ofbetween about 10 millimetres and about 20 millimetres, more preferablybetween about 12 millimetres and about 18 millimetres, more preferablybetween about 14 millimetres and about 16 millimetres, more preferablyabout 15 millimetres. Alternatively or in addition, the shreds ofhomogenised tobacco material preferably have a width of between about0.4 millimetres and about 0.8 millimetres.

Preferably, the density of the sheet of homogenised tobacco materialfrom which the shreds are formed is between about 500 and about 1500milligrams per cubic centimetre, more preferably between about 800 andabout 1200 milligrams per cubic centimetre, more preferably betweenabout 900 and about 1100 milligrams per cubic centimetre, and mostpreferably between about 900 and about 970 milligrams per cubiccentimetre.

Preferably, the bulk density of the shreds of homogenised tobaccomaterial within the aerosol-generating substrate is between about 0.4grams per cubic centimetre and about 0.8 grams per cubic centimetre,preferably between about 0.5 grams per cubic centimetre and about 0.7grams per cubic centimetre and most preferably between about 0.65 gramsper cubic centimetre and about 0.67 grams per cubic centimetre.

As described above, the homogenised tobacco material may be formed bythe casting of a slurry. Alternatively, the homogenised tobacco materialmay be formed by another suitable method, such as for example, anextrusion method.

Preferably, the aerosol-generating substrate comprises a rod of thehomogenised tobacco material circumscribed by a wrapper, wherein thewrapper is provided around and in contact with the homogenised tobaccomaterial. The wrapper may be formed from any suitable sheet materialthat is capable of being wrapped around homogenised tobacco material toform an aerosol-generating substrate. The wrapper may be porous ornon-porous. Preferably, the wrapper is a paper wrapper but the wrappermay alternatively be non-paper.

The rod of aerosol-generating substrate preferably has an externaldiameter that is approximately equal to the external diameter of theaerosol-generating article.

Preferably, the rod of aerosol-generating substrate has an externaldiameter of at least 5 millimetres. The rod of aerosol-generatingsubstrate may have an external diameter of between about 5 millimetresand about 12 millimetres, for example of between about 5 millimetres andabout 10 millimetres or of between about 6 millimetres and about 8millimetres. In a preferred embodiment, the rod of aerosol-generatingsubstrate has an external diameter of 7.2 millimetres, to within 10percent.

The rod of aerosol-generating substrate may have a length of betweenabout 7 millimetres and about 15 mm. In one embodiment, the rod ofaerosol-generating substrate may have a length of about 10 millimetres.In a preferred embodiment, the rod of aerosol-generating substrate has alength of about 12 millimetres.

Preferably, the rod of aerosol-generating substrate has a substantiallyuniform cross-section along the length of the rod. Particularlypreferably, the rod of aerosol-generating substrate has a substantiallycircular cross-section.

The aerosol-generating articles according to the invention preferablycomprise one or more elements in addition to the rod ofaerosol-generating substrate and the thermal indicator. For example,aerosol-generating articles according to the invention may furthercomprise at least one of: a mouthpiece, an aerosol-cooling element and asupport element such as a hollow acetate tube. For example, in onepreferred embodiment, an aerosol-generating article comprises, in linearsequential arrangement, a rod of aerosol-generating substrate asdescribed above, a support element located immediately downstream of theaerosol-generating substrate, an aerosol-cooling element locateddownstream of the support element, and an outer wrapper circumscribingthe rod, the support element and the aerosol-cooling element. Thethermal indicator may be provided at a defined location around the rodof aerosol-generating substrate and/or any other location around thearticle, as described above.

Aerosol-generating systems according to the present invention comprisean aerosol-generating article as described in detail above incombination with an aerosol-generating device which is adapted toreceive the upstream end of the aerosol-generating article duringsmoking. The aerosol-generating device comprises a heating element whichis configured to heat the aerosol-generating substrate in order togenerate an aerosol during use. Preferably, the heating element isadapted to penetrate the aerosol-generating substrate when theaerosol-generating article is inserted into the aerosol-generatingdevice. For example, the heating element is preferably in the form of aheater blade.

The heating element is controlled during use to operate with a definedoperating temperature range, below a maximum operating temperature. Thethermal indicator of the aerosol-generating article is adapted such thatthe threshold temperature will not be reached during normal use of theaerosol-generating article in the aerosol-generating device with theheating element operating below the maximum operating temperature. Thisensures that when the aerosol-generating article and aerosol-generatingdevice are used together, the thermal indicator will not be activatedduring normal use.

Preferably, the aerosol-generating device additionally comprises ahousing, an electrical power supply connected to the heating element anda control element configured to control the supply of power from thepower supply to the heating element.

Suitable aerosol-generating devices for use in the aerosol-generatingsystem of the present invention are described in WO-A-2013/098405.

The invention will now be further described with reference to thefigures in which:

FIG. 1 shows a perspective view of an aerosol-generating articleaccording to the present invention;

FIG. 2 shows a perspective view of an aerosol-generating article havingan ‘activated’ thermal indicator according to a first embodiment of theinvention;

FIG. 3 shows a perspective view of an aerosol-generating article havingan ‘activated’ thermal indicator according to a second embodiment of theinvention;

FIG. 4 shows a perspective view of an aerosol-generating article havingan ‘activated’ thermal indicator according to a third embodiment of theinvention;

FIGS. 5a and 5b show a schematic cross-sectional view of anaerosol-generating system comprising an aerosol-generating device and anaerosol generating article according to the invention;

The aerosol-generating article 10 shown in the figures comprises fourelements arranged in coaxial alignment: a rod of aerosol-generatingsubstrate 20, a support element 30, an aerosol-cooling element 40, and amouthpiece 50 (shown in FIGS. 5a and 5b ). Each of the four elements iscircumscribed by a corresponding plug wrap (not shown). These fourelements are arranged sequentially and are circumscribed by an outerwrapper 60 to form the aerosol-generating article 10. Theaerosol-generating 10 has a proximal or mouth end 70, which a userinserts into his or her mouth during use, and a distal end 80 located atthe opposite end of the aerosol-generating article 10 to the mouth end70.

In use air is drawn through the aerosol-generating article by a userfrom the distal end 80 to the mouth end 70. The distal end 80 of theaerosol-generating article may also be described as the upstream end ofthe aerosol-generating article 10 and the mouth end 70 of theaerosol-generating article 10 may also be described as the downstreamend of the aerosol-generating article 10. Elements of theaerosol-generating article 10 located between the mouth end 70 and thedistal end 80 can be described as being upstream of the mouth end 70 or,alternatively, downstream of the distal end 80.

The support element 30 is in place to prevent the aerosol-generatingsubstrate 20 from being forced downstream within the aerosol-generatingarticle 10 towards the aerosol-cooling element 40 when a heating elementof an aerosol-generating device is inserted into the aerosol-generatingsubstrate 20. The support element 30 also acts as a spacer to space theaerosol-cooling element 40 of the aerosol-generating article 10 from theaerosol-generating substrate 20.

The aerosol-cooling element 40 is located immediately downstream of thesupport element 30. In use, volatile substances released from theaerosol-generating substrate 20 pass along the aerosol-cooling element40 towards the mouth end 70 of the aerosol-generating article 10. Thevolatile substances may cool within the aerosol-cooling element 40 toform an aerosol that is inhaled by the user. The aerosol-cooling elementcomprises a crimped and gathered sheet of polylactic acid circumscribedby a wrapper 60. The crimped and gathered sheet of polylactic aciddefines a plurality of longitudinal channels that extend along thelength of the aerosol-cooling element 40.

The mouthpiece 50 is located immediately downstream of theaerosol-cooling element 40 and abuts the aerosol-cooling element 40. Themouthpiece 50 comprises a conventional cellulose acetate tow filter oflow filtration efficiency.

To assemble the aerosol-generating article 10, the four elementsdescribed above are aligned and tightly wrapped within the outer wrapper60. In the embodiments illustrated in the figures, the outer wrapper 60comprises conventional cigarette paper. A distal end portion of theouter wrapper 60 of the aerosol-generating article 10 is circumscribedby a band of tipping paper (not shown).

The aerosol-generating substrate 20 is located at the extreme distal orupstream end of the aerosol-generating article 10. As illustrated inFIGS. 5a and 5b , the aerosol-generating substrate 20 comprises agathered sheet of crimped homogenised tobacco material circumscribed bya wrapper. The crimped sheet of homogenised tobacco material comprisesglycerin as an aerosol former.

A thermal indicator 100 is provided at a location on the outer surfaceof the outer wrapper 60. The thermal indicator 100 comprisesthermochromic material which is configured to undergo a visual changewhen the temperature at the thermal indicator 100 is above a thresholdtemperature.

FIG. 1 shows the outer wrapper 60 of the aerosol-generating articleprior to activation of the thermal indicator 100. In the followingembodiments described herein, the thermal indicator 100 is not visibleuntil it ‘activates’ or starts to undergo visual change when thetemperature is above the threshold temperature.

FIG. 2 shows the article 10 after the activation of a thermal indicator101, according to a first embodiment, when the temperature at thethermal indicator 101 is above a threshold temperature of thethermochromic material. In this first embodiment, the thermal indicatoris located around the rod of aerosol-generating substrate 20 and thethreshold temperature is, at least, about 175° C. In the firstembodiment of the article 10, the thermal indicator 101 extendslongitudinally along the entire length of the rod of aerosol-generatingsubstrate 20. However, in other arrangements in accordance to the firstembodiment, the thermal indicator 101 may extend partiallylongitudinally along the rod of aerosol-generating substrate. Forexample, the thermal indicator 101 may extend at least 50% of the totallength of the rod of aerosol-generating substrate 20.

FIG. 3 shows the article 10 after the activation of a thermal indicator102, according to a second embodiment, when the temperature at thethermal indicator 102 is above a threshold temperature. In this secondembodiment, the thermal indicator is located partially around the rod ofaerosol-generating substrate 20 or the thermal indicator 102 does nottotally circumscribe the outer surface of the aerosol-generatingarticle. The threshold temperature of the thermal indicator 102 is, atleast, about 175° C. However, the threshold temperature for thermalindicator 102 according to the second embodiment may likely be higherthan the threshold temperature of the thermal indicator 101 according tothe first embodiment.

FIG. 4 shows the article 10 after the activation of the thermalindicator 103, according to a third embodiment, when the temperature atthe thermal indicator 103 is above a threshold temperature. In thisthird embodiment, the thermal indicator 103 is located around thearticle 10 at a position downstream of the rod of aerosol-generatingsubstrate, away from the distal end 80 of the article 10. The thermalindicator 103 may partially circumscribe the outer surface of theaerosol-generating article and may also circumscribe a portion of therod of aerosol-generating substrate. The threshold temperature of thethermal indicator 103 is, at least, about 115° C. The thresholdtemperature of the third embodiment may be lower than that of the firstand second embodiments due to the relative distance of the thermalindicator 103 from the aerosol-generating substrate 20.

As described in detail above, the visual change of the thermal indicatoris configured to inform a user of the article 10 that theaerosol-generating substrate 20 of the article 10 has been overheatedeither because the article 10 has been misused with an incompatibleaerosol-generating device or because the article has been used with anaerosol-generating device which may be malfunctioning in a manner thatit is overheating the aerosol-generating substrate 20. Consequently, inorder to convey such information of misuse of the article 10, the visualchange of the thermal indicators 101, 102, 103 are preferablyirreversible such that the visual change, upon removal of the article 10from an aerosol-generating device, does not ‘disappear’ and remainsvisible to the user when the substrate 20 has cooled. As shown in FIGS.2, 3 and 4; the irreversible visual change comprises a colour changefrom colourless to red and the appearance of warning messages orgraphical representations.

The aerosol-generating articles illustrated in the above describedfigures are designed to engage with an aerosol-generating devicecomprising a heating element in order to be consumed by a user. In use,the heating element of the aerosol-generating device heats theaerosol-generating substrate of the aerosol-generating article to asufficient temperature to form an aerosol, which is drawn downstreamthrough the aerosol-generating article and inhaled by the user.

FIGS. 5a and 5b illustrate an aerosol-generating system 500 comprisingan aerosol-generating device 510 and an aerosol-generating article 10according to any of the embodiments described above and shown in thefigures described above. It will be appreciated that theaerosol-generating device 510 could be used in combination with analternative aerosol-generating article according to the invention.

The aerosol-generating device 510 comprises a heating element 520. Asshown in FIGS. 5a and 5b , the heating element 520 is mounted within anaerosol-generating article receiving chamber 515 of theaerosol-generating device 510. In use, the user inserts theaerosol-generating article 10 into the aerosol-generating articlereceiving chamber 515 of the aerosol-generating device 510 such that theheating element 520 is directly inserted into the aerosol-generatingsubstrate 20 of the aerosol-generating article 10 as shown in FIG. 5b .In the embodiment shown in FIGS. 5a and 5b , the heating element 520 ofthe aerosol-generating device 510 is a heater blade.

The aerosol-generating device 510 comprises a power supply andelectronics (not shown) that allow the heating element 520 to beactuated. Such actuation may be manually operated or may occurautomatically in response to a user drawing on an aerosol-generatingarticle 10 inserted into the aerosol-generating article receivingchamber of the aerosol-generating device 510. A plurality of openings(not shown) may be provided in the aerosol-generating device 510 toallow air to flow to the aerosol-generating article 10.

Once the internal heating element 520 is inserted into theaerosol-generating substrate 20 of the aerosol-generating article 10 andthe heating element 520 is actuated, the aerosol-generating substrate 20of the aerosol-generating article 10 is heated to a temperature ofapproximately 350° C. by the heating element 520 of theaerosol-generating device 510. At this temperature, volatile compoundsare evolved from the aerosol-generating substrate 20 of theaerosol-generating article 10. As a user draws on the mouth end 70 ofthe aerosol-generating article 10, the volatile compounds evolved fromthe aerosol-generating substrate 20 are drawn downstream through theaerosol-generating article 10 and condense to form an aerosol that isdrawn through the mouthpiece 50 of the aerosol-generating article 10into the user's mouth.

As the aerosol passes downstream thorough the aerosol-cooling element40, the temperature of the aerosol is reduced due to transfer of thermalenergy from the aerosol to the aerosol-cooling element 40. When theaerosol enters the aerosol-cooling element 40, its temperature isapproximately 60° C. Due to cooling within the aerosol-cooling element40, the temperature of the aerosol as it exits the aerosol-coolingelement is approximately 40° C.

In FIGS. 5a and 5b , the heating element 520 of the aerosol-generatingdevice 510 is shown in a simplified manner. Particularly, the heatingelement 520 of the aerosol-generating device 510 is not drawn to scalein FIGS. 5a and 5b . Components that are not relevant for theunderstanding of the embodiment have been omitted to simplify FIGS. 5aand 5 b.

As shown in FIGS. 5a and 5b , the aerosol-generating device 510comprises a housing 530. The heating element 520 is mounted within anaerosol-generating article receiving chamber within the housing 530. Theaerosol-generating article 10 is inserted into the aerosol-generatingarticle receiving chamber 515 within the housing 530 of theaerosol-generating device 510 such that the heating element 520 isdirectly inserted into the aerosol-generating substrate 20 of theaerosol-generating article 10.

Within the housing 530 there is an electrical energy supply (not shown),for example a rechargeable lithium ion battery. A controller (not shown)is connected to the heating element 520, the electrical energy supply,and a user interface (not shown), for example a button or display. Thecontroller of the aerosol-generating device controls the power suppliedto the heating element 520 in order to regulate its temperature.

During this normal usage of the aerosol-generating articles according tothe invention with the compatible aerosol-generating device 510 shown inFIGS. 5a and 5b , the thermal indicator of the aerosol-generatingarticle remains ‘inactivated’ due to the use of the article with acompatible aerosol-generating device and, as such, overheating does notoccur in the aerosol-generating substrate.

In the event that the aerosol-generating articles according to theinvention are used with an incompatible device and are overheated toabove a preferred maximum operating temperature, the thermal indicatorwill ‘activate’, that is undergo a visual change, upon reaching thepredetermined threshold temperature of the thermal indicator. Uponremoval of the article from the aerosol-generating device, the user willbe informed of the misuse of the article when she sees the ‘activated’thermal indicator of the articles.

As such, the incorporation of the thermal indicator into theaerosol-generating articles of the present invention provides aneffective means of protecting the consumer from use of theaerosol-generating article in an unintended manner in an incompatibledevice and/or from use of the article in a defective or malfunctioningcompatible device.

1.-15. (canceled)
 16. An aerosol-generating article for anaerosol-generating device having a heating element, theaerosol-generating article comprising: a rod of aerosol-generatingsubstrate, the rod of aerosol-generating substrate being formed of anaerosol-forming material having an aerosol former content of between 5percent and 30 percent on a dry weight basis; and an outer wrapper,wherein the outer wrapper is wrapped around at least a portion of therod of aerosol-generating substrate and at least a portion of an outersurface of the wrapper comprises a thermal indicator, the thermalindicator comprising: a thermochromic material, whereby at least aportion of the thermochromic material is configured to undergo a visualchange when a temperature at said portion of the thermochromic materialexceeds a threshold temperature, such that the outer wrapper provides avisual indication that a temperature at the thermal indicator hasexceeded the threshold temperature, wherein the threshold temperature isat least about 175 degrees Celsius.
 17. An aerosol-generating articlefor an aerosol-generating device having a heating element, theaerosol-generating article comprising: a rod of aerosol-generatingsubstrate; and an outer wrapper, wherein the outer wrapper is wrappedaround at least a portion of the rod of aerosol-generating substrate andat least a portion of an outer surface of the wrapper comprises athermal indicator, the thermal indicator comprising: a thermochromicmaterial, whereby at least a portion of the thermochromic material isconfigured to undergo a visual change when a temperature at said portionof the thermochromic material exceeds a threshold temperature, such thatthe outer wrapper provides a visual indication that a temperature at thethermal indicator has exceeded the threshold temperature, wherein thethreshold temperature is at least about 175 degrees Celsius, wherein thethermal indicator overlies at least 5 square millimetres of an externalsurface area of the rod of aerosol-generating substrate.
 18. Anaerosol-generating article for an aerosol-generating device having aheating element, the aerosol-generating article comprising: a rod ofaerosol-generating substrate; and an outer wrapper, wherein the outerwrapper is wrapped around at least a portion of the rod ofaerosol-generating substrate and at least a portion of an outer surfaceof the wrapper comprises a thermal indicator, the thermal indicatorcomprising: a thermochromic material, whereby at least a portion of thethermochromic material is configured to undergo a visual change when atemperature at said portion of the thermochromic material exceeds athreshold temperature, such that the outer wrapper provides a visualindication that a temperature at the thermal indicator has exceeded thethreshold temperature, wherein the threshold temperature is at leastabout 220 degrees Celsius.
 19. The aerosol-generating article accordingto claim 16, wherein the threshold temperature is at least about 220degrees Celsius.
 20. The aerosol-generating article according to claim16, wherein the thermal indicator overlies at least 5 square millimetresof an external surface area of the rod of aerosol-generating substrate.21. The aerosol-generating article according to claim 16, wherein thethermal indicator extends longitudinally along at least 50 percent of atotal length of the rod of aerosol-generating substrate.
 22. Theaerosol-generating article according to claim 16, wherein thethermochromic material is arranged such that the visual change occursonly within a circumferential portion of the outer wrapper, and whereinthe circumferential portion extends only partially around acircumference of the aerosol-generating article.
 23. Anaerosol-generating article for an aerosol-generating device having aheating element, the aerosol-generating article comprising: a rod ofaerosol-generating substrate, the rod of aerosol-generating substratebeing formed of an aerosol-forming material having an aerosol formercontent of between 5 percent and 30 percent on a dry weight basis; andan outer wrapper, wherein at least a portion of an outer surface of thewrapper comprises a thermal indicator, wherein the thermal indicator isdisposed downstream of the rod of aerosol-generating substrate, thethermal indicator comprising: a thermochromic material, whereby at leasta portion of the thermochromic material is configured to undergo avisual change when a temperature at said portion of the thermochromicmaterial exceeds a threshold temperature, such that the outer wrapperprovides a visual indication that a temperature at the thermal indicatorhas exceeded the threshold temperature, wherein the thresholdtemperature is at least about 115 degrees Celsius.
 24. Theaerosol-generating article according to claim 16, wherein the visualchange is irreversible.
 25. The aerosol-generating article according toclaim 16, wherein the thermal indicator extends longitudinally along atleast 20 percent of a total length of the aerosol-generating article.26. The aerosol-generating article according to claim 16, wherein thevisual change comprises a colour change.
 27. The aerosol-generatingarticle according to claim 16, wherein the visual change comprisesdisplaying a graphical representation and/or warning indicia ormessages.
 28. The aerosol-generating article according to claim 16,wherein the outer wrapper comprises temperature indicating sensor paper,whereby the thermochromic material is integral to the temperatureindicating sensor paper.
 29. The aerosol-generating article according toclaim 16, wherein the thermochromic material comprises thermochromicpaint, ink, or dye applied to the at least a portion of the outersurface of the wrapper.
 30. The aerosol-generating article according toclaim 16, further comprising: a mouthpiece disposed downstream of therod of aerosol-generating substrate; a support element disposedimmediately downstream of the rod of aerosol-generating substrate; andan aerosol-cooling element disposed between the support element and themouthpiece.
 31. A thermochromic material in a thermal indicator disposedon an outer wrapper of a rod of aerosol-generating substrate of anaerosol-generating article, the rod of aerosol-generating substratebeing formed of an aerosol-forming material having an aerosol formercontent of between 5 percent and 30 percent on a dry weight basis andthe aerosol-generating article being configured to be used with anaerosol-generating device having a heating element, wherein the outerwrapper is wrapped around at least a portion of the rod ofaerosol-generating substrate and whereby at least a portion of thethermochromic material is configured to undergo a visual change when atemperature at said portion of the thermochromic material exceeds athreshold temperature, such that the outer wrapper provides a visualindication that a temperature at the thermal indicator has exceeded thethreshold temperature, wherein the threshold temperature is at leastabout 175 degrees Celsius.
 32. A thermochromic material in a thermalindicator disposed on an outer wrapper of an aerosol-generating article,the aerosol-generating article being configured to be used with anaerosol-generating device having a heating element, and theaerosol-generating article comprising a rod of aerosol-generatingsubstrate, the rod of aerosol-generating substrate being formed of anaerosol-forming material having an aerosol former content of between 5percent and 30 percent on a dry weight basis, whereby the thermalindicator is disposed downstream of the rod of aerosol-generatingsubstrate and at least a portion of the thermochromic material isconfigured to undergo a visual change when a temperature at said portionof the thermochromic material exceeds a threshold temperature, such thatthe outer wrapper provides a visual indication that a temperature at thethermal indicator has exceeded the threshold temperature, wherein thethreshold temperature is at least about 115 degrees Celsius.
 33. Anaerosol-generating system, comprising: an aerosol-generating articleaccording to claim 16; and an aerosol-generating device configured toreceive the aerosol-generating article, the aerosol-generating devicecomprising a heater element configured to heat a rod ofaerosol-generating material, wherein the heater element is configured soas to be controlled to operate below a maximum operating temperature,wherein a thermal indicator of the aerosol-generating article isconfigured such that a threshold temperature is not exceeded during useof the aerosol-generating system with the heater element operating belowthe maximum operating temperature.